1. Field of the Invention
This invention relates generally to a spread spectrum telecommunications system and, more particularly, to a channel element modem in a CDMA cellular communication system.
2. Description of the Related Art
To serve a large number of telecommunication users over a limited electro-magnetic spectrum, one of a variety of multiple access techniques may be used. These techniques include, for example, time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). The CDMA technique has many advantages over the other techniques, and an exemplary CDMA system is described in U.S. Pat. No. 4,901,307 issued Feb. 13, 1990, assigned to the assignee of this invention, and incorporated herein by reference.
In the CDMA cellular system described in the '307 patent, a large number of mobile telephone system users, each having a transceiver, communicates through satellite repeaters or terrestrial stations which are also referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographic area and routes calls carried over cellular telephones to and from a telecommunication network such as a public switched telephone network (PSTN). When a cellular telephone user moves into the geographic area of a new cell, the routing of that user's call may be eventually made through the new cell by a process called "handing off".
A cellular telephone or, more specifically, a mobile unit, broadcasts a signal that is received by a cell and then is routed to the PSTN and to telephone lines or other mobile units. A cell broadcasts a cell signal that is received by mobile units. The cell-to-mobile signal transmission path is generally referred to as the "forward link" and the mobile-to-cell transmission path is generally referred to as the "reverse link".
The CDMA technique permits a frequency spectrum to be effectively used multiple times within a time interval, thus increasing system user capacity. The CDMA technique described in the '307 patent makes use of high-frequency pseudo-noise (PN) code modulation of individual calls, as well as modulation by codes including orthogonal binary sequences, to combine many calls and broadcast them as a single CDMA signal. In this way, the CDMA technique permits discrimination between many calls that occupy the same frequency band and provides increased spectral efficiency as compared with other techniques.
A system and method for CDMA telephone system communications is described in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992 and in U.S. Pat. No. 5,267,261, issued Nov. 30, 1993, both assigned to the assignee of the present invention, and both incorporated herein by reference. These patents both disclose base station architecture and operation. The following co-pending U.S. Patent Applications, assigned to the assignee of this application, describe modulator-demodulator (MODEM) architectures that implement a complete CDMA base band modem that performs reverse link demodulation and forward link modulation:
1. U.S. patent application Ser. No. 08/316,156, filed Sep. 30, 1994, for "SERIAL LINKED INTERCONNECT FOR SUMMATION OF MULTIPLE WAVEFORMS ON A COMMON CHANNEL", inventors K. Easton et al.; PA1 2. U.S. patent application Ser. No. 08/316,177, filed Sep. 30, 1994, for "MULTIPATH SEARCH PROCESSOR FOR A MULTIPLE ACCESS COMMUNICATION SYSTEM", inventors K. Easton et al.; PA1 3. U.S. patent application Ser. No. 08/372,632, filed Jan. 13, 1995, for "CELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM", inventors K. Easton et al.; and PA1 4. U.S. patent application Ser. No. 08/492,592, filed Jan. 20, 1995, for "MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM", inventors K. Easton et al.
The main thrust for development of the physical modem architecture described in detail in these co-pending applications has been to provide reduction in component count and cost. Assuming implementation of a base station modem architecture in a single application specific integrated circuit (ASIC), there is a concurrent need for an accompanying processing component that supports the physical implementation of modem functions in ASIC.
At the heart of a CDMA system is a physical layer embracing a number of forward and reverse CDMA channels. In this regard, a forward CDMA channel is a channel from a base station to mobile stations and includes one or more code channels. Code channels are subchannels of forward CDMA channels that are assigned for specific purposes and distinguished by orthogonal coding. A reverse CDMA channel is a code channel from a mobile station to a base station. Preferably, a channel element in a CDMA base station is a subsystem comprising hardware and software that supports one forward and one reverse code channel. Each channel element of a plurality of channel elements in a base station includes a modem ASIC, and a channel element software program to implement control of the physical structure's channel operations.